This invention relates in general to a drawn and ironed can, and more particularly to a drawn and ironed can body having an improved end wall configuration.
The so-called drawn and ironed can has to a large measure replaced the old three piece can, at least in the beverage industry. Moreover, these cans are made almost exclusively from aluminum, which being quite ductile, is easily drawn into a cylindrical configuration and ironed down to a very thin wall thickness. While the economies mass production are reflected in the low cost of the cans, the cost of the sheet aluminum from which the cans are manufactured has nevertheless always been an important consideration. Through the years various advances in can technology have enabled the can bodies to be manufactured from thinner and thinner aluminum sheet.
The typical drawn and ironed can consists of two components, namely a top and a can body. Only the latter is formed by a drawing and ironing procedure, and when completed it includes a very thin side wall and a domed end wall formed integral with the side wall at one end of the side wall. The opposite end of the side wall is joined to the top along a seam, but only after a beverage is introduced into the can body.
To form the can bodies, circular disks are first stamped from aluminum sheet stock of the appropriate thickness. This, of course, results in a considerable amount of scrap. Next, each disk is drawn into a cup. The cup is then placed over the end of a punch and forced through a die set where it is redrawn into a lesser diameter and ironed along its side wall to substantially reduce the thickness of the side wall while at the same time elongating the side wall. The end wall, however, retains the original thickness of the sheet stock, and after the side wall is completely ironed, the punch drives the end wall against an end forming die to impart a domed configuration and surrounding rim to it. This configuration enables the end wall to withstand high internal pressures without buckling outwardly and rendering the can unstable, and further gives it adequate column strength.
However, the use of thinner stock reduces the strength of the domed end wall, and even a slight reduction in thickness will cause a can having the conventional end wall profile to buckle outwardly under elevated pressures, such as the pressures that may be encountered during the pasteurization of beer. In other words, the external surface of the end wall changes from a concave configuration to a convex configuration, and when this occurs the can will not rest in a stable upright position on a horizontal surface. This may cause the can to topple during subsequent handling in the brewery and thereby disrupt equipment, and furthermore a buckled end wall destroys the appearance of the product. Moreover, cans with conventional end wall profiles have very little capacity for accommodating overfill without buckling the end wall.